SALT LAKE CITY  Federal scientists studying the Great Salt Lake have found some of the highest levels of mercury ever measured anywhere -- prompting concern about some of the migratory birds that feed on the lake's brine shrimp.

U.S. Geological Survey and Fish and Wildlife Service researchers were initially gathering information on selenium in the lake, but decided also to test the samples for mercury.

Concentrations of methylmercury -- the element's most poisonous form -- exceeded 25 nanograms per liter of water. Fish consumption warnings have been issued when there was just 1 nanogram per liter.

"We thought we would find some high levels of methylmercury," said David Naftz, the USGS research hydrologist who is heading the Great Salt Lake project, "but not some of the highest (the USGS) has ever found."

There are no fish in the Great Salt Lake, and no evidence yet that mercury from the lake is getting into the human food chain. But the brine shrimp the project scientists have studied show evidence of mercury buildup that could be harmful to the lake's migratory birds.

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The bird they studied is the eared grebe, which eats brine shrimp from May to December. The researchers found mercury levels in the birds' livers more than doubled during their months on the lake.

The study's preliminary findings eventually may overturn the long-held idea that areas of the lake's deep brine layer, which has no oxygen, is a kind of disposal system where toxins sink to the lake bed and become inert. Instead, the USGS study suggests the lake's peculiar chemistry actually speeds the conversion of mercury to its more toxic organic form.

"It's not a disposal, it's a factory," Naftz said.

Mercury is a highly toxic element that occurs naturally in the environment but also has been introduced through mining and industrial activity.

Though the USGS studies have not found any evidence that mercury in the Great Salt Lake has entered the human food chain, ducks and geese that feed in the lake's wetlands could be subject to the same accumulation found in the eared grebes, said Fish and Wildlife researcher Bruce Waddell.

People who eat the waterfowl might be exposed to mercury, agreed Steven Schwarzbach, a research manager for the USGS at the Western Ecological Research Center in Sacramento, Calif. The Great Salt Lake study hasn't yet tested that possibility, however.

"There are some mysteries out here that are just starting to be discovered, so it's not fair to make an assessment of what's happening," Waddell said.